1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a paper guide device for guiding paper fed to a nip, between an image belt and a transfer roller, and an image forming apparatus including the same.
2. Description of the Related Art
Generally, an image forming apparatus such as an electro-photographic printer forms images on a recording medium, such as sheets of paper, by forming an electrostatic latent image on a photoconductor. The photoconductor may be an organic photoconductive (OPC) drum or a photoconductive belt for developing the formed electrostatic latent image by using a developer having a predetermined color and transferring the developed image to the recording medium. The electro-photographic printer is classified as a wet type or a dry type according to the type of developer being used. In the wet type printer, a liquid developer is used for developing electrostatic latent images formed on the photoconductor. The liquid developer is a mixture of a liquid carrier and a powder type of a toner.
FIG. 1 is a diagram illustrating a conventional wet type color electro-photographic printer.
As shown in FIG. 1, the conventional wet type color electro-photographic printer 1 includes an image forming unit 5, a transfer belt unit 10, a convey guiding unit 40, a fixing unit 21, a paper discharging unit 30 and a cleaning unit 50.
Each image forming unit 5 includes a laser scanning unit 11, a charge roller 12, a photoconductor 9 and a developing device 13 for forming an image with colors typically including black, yellow, cyan and magenta.
Each of the developing devices 13 includes a developer storage unit 6, a developing roller 7, a deposit roller 14, a metering roller 15 and a cleaning roller 16. The developer storage unit 6 stores a liquid developer 48. The developing roller 7 is arranged below the photoconductor 9. The deposit roller 14 is arranged below the developing roller 7. The deposit roller 14 forms a developer layer on the developing roller 7 by supplying electrical power to the liquid developer 48. The metering roller 15 provides a predetermined voltage to the developer layer on the developing roller 7 and controls the developer layer to maintain a constant amount of toner or constant concentration (solid %). After controlling, the metering roller 15 provides the controlled developer layer to a nip between the developing roller 7 and the photoconductor 9. The cleaning roller 16 cleans the developing roller 7.
The deposit roller 14 and the metering roller 15 supply the developing layer, having a constant amount of toner or constant concentration, to the nip between the developing roller 7 and the photoconductor 9. This is done regardless of the concentration of 30-40 solid % of high concentration liquid developer, or liquid developer 48 which varies during use of the liquid developer 48.
Each of photoconductors 9 is formed of an organic photoconductive (OPC) drum. Different colors of toner images are formed on the photoconductors 9 by corresponding developing devices 13.
The transfer belt unit 10 includes a first transfer roller 8, a second transfer roller 23 and an image transfer belt 17. The image transfer belt 17 rotates along a path of a caterpillar track made by a first, a second and a third supporting rollers 19, 20 and 21. The first transfer roller 8 overlappingly transfers toner images formed on the photoconductors 9 to the image transfer belt 17. The second transfer roller 23 transfers the toner images formed on the image transfer belt 17 to sheets of paper P.
A paper guide unit 35 includes a convey guiding unit 40 for guiding paper to a nip between the image transfer belt 17 and the second transfer roller 23. The paper is fed to the transfer belt unit 10 by a convey roller (not shown) of a paper feeding unit (not shown). The convey guiding unit 40 is fixed to a fixing bracket 44 of a movable frame (not shown) for supporting a shaft 23a of the second transfer roller 23, as shown in FIG. 2.
Also as shown in FIG. 2, the convey guiding unit 40 includes a first guide plate 43 and a second guide plate 41. The first guide plate 43 guides a back side of the paper, where none of the images are formed, and is fixed to the fixing bracket 44. The second guide plate 41 guides a front side of the paper, where images are formed, and is fixed to the first guide plate 43. The second guide plate 41 is arranged within a predetermined space from side guide plate 45. The side guide plate 45 is arranged on both sides of the first guide plate 43 and formed on the fixing bracket 44.
The fixing unit 21 includes first and second heating rollers 25 and 27, and first and second pressuring rollers 26 and 28. The first and the second heating rollers 25 and 27 provide heat to the transferred toner image on the paper P. The first and second pressuring rollers 26 and 28 pressurize the paper P with a constant pressure. The heated and pressurized paper P, with the toner image, is discharged to the exterior of the unit by first and second paper feeding roller 31 and 32. First and second paper feeding backup rollers 33 and 34 are also arranged in the paper discharging unit 30.
The cleaning unit 50 includes a cleaning blade 51 for removing used developer remaining on the image transfer belt 17 after transferring the toner image on the paper P and a used developer storage 52 for storing the removed developer.
Hereinafter, operations of the conventional wet type color electro-photographic printer 1 are explained in detail.
When the conventional wet type color electro photographic printer 1 receives a printing command, the image forming unit 5 performs an image forming operation for forming images with four colors.
That is, an electrostatic latent image is formed on each of photoconductors 9 by using the charge roller 12 and the laser scanning unit 11. After forming the electrostatic latent images, the deposit roller 14 and the metering roller 15 form a toner image by attaching toner of a developer layer to the electrostatic latent image on the photoconductor 9 by using the liquid developer 48 stored in the storing unit 6.
The liquid developer 48 is formed as a charged developer layer on the developing roller 17 by electric power from the deposit roller 14. The metering roller 15 supplies a predetermined voltage to the charged developer layer and regulates it to have a constant amount of toner.
The toner images formed on the photoconductors 9 are overlappingly transferred to the image transfer belt 17 by voltage and pressure from the first transfer roller 8. After transferring the toner images on the image transfer belt 17, a belt operating roller 22 rotates the image transfer belt 17 along the first, the second and the third supporting rollers 19, 20, 21 for moving the image transfer belt 17 to the second transfer roller 23. When the image transfer belt 17 reaches the second transfer roller 23, the overlapped toner image formed on the image transfer belt 17 is transferred to the paper P by voltage and pressure from the second transfer roller 23. The paper P is conveyed from the paper feeding unit to the transfer belt unit 10 through the convey guiding unit 40.
The toner image-transferred paper is heated and pressurized by the first and the second heating rollers 25 and 27, and the first and the second pressuring roller 26 and 28.
After firmly fixing the overlapped toner image onto the paper P, the paper P is discharged from the printer by the first and the second paper discharging rollers 31 and 32, and the first and second paper discharging backup rollers 33 and 34 in the paper discharging unit 30.
After transferring the overlapped toner image on the paper P, the image transferring belt is continuously rotated and moved to the cleaning blade 51. The cleaning blade 41 removes developer remaining on a surface of the image transfer belt 17 and the used developer storage unit 52 collects the removed developer. Generally, during transferring, 90-98% of developer is transferred from the image transfer belt 17 to the paper and 2-10% of developer remains on the image transfer belt 17 without being transferred thereto.
After removing the remaining developer, the above mentioned operations are repeatedly performed for forming another image onto the paper.
The conventional wet type color electro photographic printer 1 guides the paper P to the nip between the image transfer belt 17 and the second transfer roller 23. This is accomplished by using the convey guiding unit 40 when the paper is fed to the transfer belt unit 10 for transferring the toner image formed on the image transfer belt 17 to the paper P.
However, the paper P is not directly fed to the nip between the image transfer belt 17 and the second transfer roller 23. As shown in FIG. 2, first an upper edge of the paper P is guided to a surface of the image transfer belt 17. That is, the upper edge of the paper directly contacts an image forming surface of the image transfer belt 17 before the paper P is fed to the nip. Accordingly, the image forming surface of the image transfer belt 17 is impacted by a shock from the upper edge of the paper when the paper contacts to the image transfer belt 17.
As a result, since the image transfer belt 17 is generally manufactured by a gummous material, a number of scratches are formed on the image forming surface of the image transfer belt 17, due to the impact of the upper edge of the paper. Accordingly, the quality of the toner image transferred from the image transfer belt to the paper P degrades.
The first and the second guide plates 43 and 41 are spaced apart from the image transfer belt 17 because the image transfer belt 17 vibrates when the image transfer belt 17 is rotated by the belt operating roller 22.
Accordingly, the upper edge of the paper P is not completely guided to the nip when the paper is fed to the transfer belt unit 10 through the first and the second guide plates 43 and 41. Therefore, the paper P often curls at an entrance of the nip and frequently causes paper jams.
Therefore, there have been many attempts to develop a paper guide device for preventing the image transfer belt 17 from being damaged by the upper edge of the paper P and preventing paper jams from being generated between the image transfer belt 17 and the second transfer roller 23 when the paper is fed to the transfer belt unit 10.
Accordingly, there is a need for an improved paper guide device for accurately guiding sheet of paper to a nip between an image transfer belt and a transfer roller while preventing damage to the image transfer belt.